Schaller Rotor Sound

Some like 'em and some don't, but I bet everyone has at least a couple — Analog Delay, Digital Delay, Reverb, Chorus, Flanger, Phaser, Tube Overdrive, Transistor Overdrive, Fuzz, Octave Fuzz, Tremolo, Vibrato, Compressor and the list goes on!

Re: Schaller Rotor Sound

Postby zigomar on Thu Aug 08, 2013 1:20 pm

Hi there,
i just found one of this fabulous effect box in a scrap. Mint state!!
After a smooth cleaning, i test it.
Incredible sound, like fast liquid bubbles…

The mysterious roud metallic box contains, according to the schematics, only a 6v 0.1A light bulb on top and 2 Light sensors (LDR03) on bottom.
The "speed" pot looks affects only the brightness of the bulb…
By transition from AC 6v/50Hz to DC 6v through the diode bridge?? :shock:
Don't ask me how it works… But, 45 years later, it works very well!
Some people told me i have to replace the chemical capacitors…
Why? I love that "LowFi" noise! :D

Happy Hippie
Last edited by zigomar on Sun Aug 11, 2013 7:04 pm, edited 2 times in total.
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Re: Schaller Rotor Sound

Postby vez on Sun Aug 11, 2013 3:05 am

I wonder if this is what John Paul Jones used in Led Zeppelin's "No Quarter"
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Re: Schaller Rotor Sound

Postby zigomar on Sun Aug 11, 2013 8:10 pm

I wanted to post a sound sample, but the .mp3 .wav .aif .mp4 are not allowed on this site! How to do? :(
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Re: Schaller Rotor Sound

Postby General Electron on Mon Mar 10, 2014 10:10 pm

I'll have a go at guessing how this weird beast works.

The signal passes through a chain of 18 transformers, but not in any rational connection (involving applying the signal to the primary winding, and getting a signal out of the secondary winding.) Rather, it is just used as a chain of inductors made up of just the primaries of the transformers, with capacitors to ground between all the inductors. This long chain of L's and C's acts as a phase shifter. For fixed values of C and L, a fixed phase shift as a function of frequency is obtained. For further reading, search on "transmission lines", and perhaps also look at schematics of the Hammond organ vibrato/chorus unit. For some values of L and C, all the phase shift sections will have their phase at the full -180 degrees by the lowest frequency of the audio range, and then this circuit becomes a constant time delay. Not all that long a delay - but it doesn't have to be. I don't know what the turnover frequency of the all pass sections will be in this circuit, since the inductance L is unknown; it is possible the phase is still changing throughout the audio range. Might even be better that way.

But a static phase shift does not make for a very interesting effect. The LFO - which totally eludes me as to how it works, but I don't believe that is important here - drives the transformer secondaries with a slowly varying voltage. If the voltage is large enough, the iron cores of the transformers will begin to saturate. At that point, the inductance seen by the primary side will begin to decrease. So now we get a variable delay/phase shifter, with the LFO controlling the amount of delay, which should be a familiar concept.

There certainly were other technologies available when this product was built, but this technique probably sounds different from anything else. I'm not sure if there could be some distortion added from the core saturation.

I would like to see someone try to build a version of this. Personally, I'm too busy writing DSP code these days. I suggest using some small and cheap 1:1 audio transformers. You want them small so that the iron cores will saturate with a reasonable current - before the tiny wires melt. I found some surplus parts labelled "coupling transformers" that are 600 ohm primary and 365 ohm secondary for only $1 that might work.

Plan 9 - revival of the dead technology. Have at it.
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